Electrophoretic displays typically use reflective and absorbing pigments to produce optical performance that dramatically differs from traditional electronic displays. The white pigments, for example, used in the electrophoretic displays typically reflect light by a multiple scattering mechanism. The pigments thus are isotropic diffusers that create a Lambertian distribution of light reflected from a pigment surface. Electrophoretic displays thus typically have a light output intensity that approximates a Lambertian distribution. The output of reflective liquid crystal displays, in contrast, has an intensity that varies substantially with viewing direction.
Certain optical inefficiencies exist, however, in the typical electrophoretic display. In a typical electrophoretic display, a sizable fraction of light scatters from the reflective pigment at a steep angle relative to the normal to the scattering surface. A large portion of this scattered light then experiences internal reflection at the interface between the ambient air and the front surface of the display. After internal reflection, the light has only a small chance of being “recycled” (i.e., reflected in a manner so as to be capable of being seen by an observer of the display), through re-scattering by a nearby pixel.
Internal reflection inefficiency can cause the brightness of the electrophoretic display to be reduced by up to 50% or more. Furthermore, re-scattering from neighboring pixels can cause undesirable optical cross-talk.